1. Field of the Invention
The present invention relates to a Switching Mode Power Supply (hereinafter, referred to as “SMPS”). More particularly, the present invention relates to a SMPS of a quasi-resonant switching mode and a driving method thereof.
2. Description of the Related Art
The SMPS is a device for rectifying an input AC voltage to an input DC voltage (DC-Link voltage), and transforming the input DC voltage into a DC output voltage having a different level. SMPSs can be generally used in power electronic devices, for example, battery power supplies, such as mobile phones and laptop computers.
In general, the SMPS shows great conduction loss when an input voltage is lower than a specific level and small conduction loss when an input voltage is higher than a specific level. However, in this latter case the SMPS has an increased switching loss, making it difficult to improve its efficiency.
To solve the problem, a method of driving the SMPS in continuous conduction mode (hereinafter, referred to as “CCM”) or discontinuous conduction mode (hereinafter, referred to as “DCM”) has been proposed.
FIG. 1 is a graph illustrating the relationship between an input voltage and efficiency when the SMPS is driven in the CCM or DCM of the prior art. When the SMPS is driven in the CCM, the conduction loss is advantageously small when an input voltage is low, but the switching loss disadvantageously increases and efficiency is degraded when the input voltage increases or an output terminal load decreases. In contrast, when the SMPS is driven in the DCM, the switching loss can be reduced when an input voltage increases or an output terminal load decreases, since a transistor is turned on using a quasi-resonant switching mode when a drain voltage of the main switch of the SMPS is the lowest. However, when the input voltage is low, the conduction loss is greater in the DCM than in the CCM.
Therefore, there has been a need for a new driving method capable of improving driving efficiency of the conventional SMPS in the quasi-resonant switching mode.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.